Impact of Layer Stacking Manner on the Lithium-Ion-Battery Performance in Electrically Neutral Tetraoxolene-Bridged Iron(II) Hexagonal Layer Metal–Organic Frameworks

Cathode materials for lithium-ion battery (LIB) cells
present a
fascinating target for applications in metal–organic frameworks
(MOFs). While the framework moiety of MOFs acts as an electron container
involving redox reactions, the pores store Li+ ions in
MOF-incorporating LIBs. Thus, in addition to the redox potential and
electron-conjugating nature of the framework, the path features for
Li+-ion migration between the frameworks are closely associated
with the LIB performance. Herein, we demonstrate the impact of porosity
on the LIB performance using a series of charge-neutral layered MOFs,
[FeII2(X2An)2(bpym)] (X
= F, 1; Cl, 2; Br, 3; X2An2– = 2,5-dihalogeno-3,6-dihydroxy-1,4-benzoquinonate;
bpym = 2,2′-bipyrimidyl), as cathode materials for LIBs. All
compounds have a similar layered structure with the same electronic
state; however, 1 has an eclipsed layer-stacking, whereas
the isostructural 2 and 3 have polymorphic
staggered structures, which results in one-dimensional channel paths
in 1 and isolated pores in 2 and 3. The battery capacity was not dependent on the compound at low current
densities but was largely affected by the stacking manner at high
current densities and overpotentials: owing to the 1D channel that
offers a good diffusion path for lithium ions, 1 exhibited
desirable characteristics for LIBs.

锂离子电池（LIB）用阴极材料在金属-有机框架（MOFs）领域的应用中展现出令人着迷的前景。MOFs的框架部分充当电子容器，涉及氧化还原反应，而孔道则储存MOFs中嵌入的LIB的Li+离子。因此，除了框架的氧化还原电位和电子共轭性质外，Li+-离子在框架之间迁移的路径特征与LIB的性能密切相关。本研究中，我们通过一系列电荷中性层状MOFs [FeII2(X2An)2(bpym)]（X = F, 1; Cl, 2; Br, 3; X2An2– = 2,5-二卤代-3,6-二羟基-1,4-苯醌酸；bpym = 2,2′-联吡啶）作为LIB的阴极材料，展示了孔度对LIB性能的影响。所有化合物均具有相似的层状结构以及相同的电子状态；然而，1号化合物具有重叠的层状堆积结构，而同构的2号和3号化合物则具有多晶错位的结构，这导致1号化合物中存在一维通道路径，而2号和3号化合物中则存在孤立的孔道。在低电流密度下，电池容量不受化合物影响，但在高电流密度和过电位下，电池容量主要受堆积方式的影响：由于1D通道提供了锂离子良好的扩散路径，1号化合物表现出对LIB有利的特性。